The present invention relates to techniques for providing inhalation gases to patients during experimental, clinical, and analytical studies and, more particularly, to methods and apparatus for conducting relatively high concentration enhanced gas studies utilizing a closed loop system.
Since at least the 1950s, neurologists have recognized the significance of cerebral blood flow for evaluating brain functions, e.g., cerebral metabolic rates. Low-concentration radioactive xenon inhalation studies have long been utilized to provide analysis of matter flow within multiple cerebral and brain-stem regions. The inability to measure blood tissue coefficients, poor resolution due to tissue overlap, and contamination by extracranial blood flow have, however, reduced the acceptance of radioactive xenon inhalation techniques for cerebral blood flow mapping.
In the late 1970s, neurologists began to explore the possibility of using relatively high concentration non-radioactive xenon gas to measure cerebral blood flow. Stable xenon gas freely passes the blood/brain barrier, and the heavy gas (Atomic No. 54) attentuates X-rays. As rapid, sequential transmission computed tomography (CT) scanners with a high signal-to-noise ratio became available, interest in xenon-enhanced inhalation studies heightened. Xenon-enhanced inhalation CT studies are currently utilized or considered for a wide range of experimental, clinical, and analytical tests, including tests to differentiate coma from brain death, studies for patients suffering from dementias and multiple sclerosis, and studies for patients experiencing trauma, vascular spasms, and seizures.
The expense of the xenon gas for such studies has, however, minimized research regarding and employment of xenon-enhanced/CT techniques. No universally acceptable method for conducting xenon-enhanced/CT studies exists, and therefore any efforts to minimize xenon gas costs must be compatable With various techniques. ln general, xenon concentrations in end-tidal gas are assumed to be proportional to xenon concentrations in arterial blood, and xenon end-tidal gas concentrations are therefore an input function in the determination of cerebral blood flow. One technique for determining end-tidal concentrations is to utilize the "subtraction method", which requires patient exposure to 100% oxygen in order to obtain substantially total denitrogenation prior to xenon inhalation. Other common techniques for directly measuring end-tidal xenon gas concentrations utilize a mass spectrometer or a thermal conductivity detector.
Further variations regarding the procedure for conducting xenon inhalation studies depend on the particular desires of the neurologists and needs of the patient. Neurologists desires may vary from a relatively low 28% xenon concentration to a relatively high 40% xenon concentration, although 35% xenon concentration is a commonly-recognized norm. Oxygen inhalation concentrations will obviously depend upon the particular needs of the patient, and may increase from the norm of 21% to 50% or more for patients requiring increased oxygen levels, or to 100% for the pre-xenon inhalation period required to obtain patient denitrogenation.
Xenon-enhanced inhalation periods for cerebral blood flow analysis studies typically vary from about 4 to 7 minutes and those skilled in the art recognize that the cost of the xenon gas inhaled during such studies is a factor detrimental to xenon/CT acceptance in the industry. Although cost estimates vary, it is generally presumed that the xenon cost to patients for a single study may be in the range of from $50 to $150. Moreover, continual variations in xenon/CT techniques and data analysis experiments must be widely performed on test animals, such as baboons, prior to widespread utilization and acceptance of this technology in the medical industry. Xenon usage is therefore a significant cost to both experimental and clinical xenon-enhanced inhalation studies. Further background regarding xenon-enhanced cerebral blood flow studies may be obtained from the following articles: "Mapping Local Blood Flow Of Human Brain By CT Scanning During Stable Xenon Inhalation", by Meyer et al, STROKE, Vol. 12, No. 4, pp. 426-436, July-August 1981; "Xenon and CT Provide Cerebral Blood Flow Measure", DIAGNOSTIC IMAGING, September 1984, pp. 13-14; "Simultaneous Mass Spectrometry and Thermoconductivity Measurements of End-Tidal Xenon Concentrations: A Comparison", by Gur et al, MED PHYS, Vol. 11, No. 2, March-April 1984, pp. 209-212; and "Mapping Cerebral Blood Flow By Xenon Enhanced Computed Tomography: Clinical Experience", by Yonas et al, RADIOLOGY, Vol. 152, No. 2, August 1984, pp. 435-442.
The prior art does not provide an acceptable technique for substantially reducing xenon costs for such tests, while simultaneously providing a technique compatible with the various CT procedures. The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter described for performing xenon-enhanced studies. The method and apparatus of the present invention may also be used to reduce the cost of various other gases utilized in inhalation-related procedures.